Head, Child and Family Research, National Institute of Child Health and Human Development, U.S. Department of Health and Human Services, Bethesda, Maryland. Author of Development in Infancy: An Introduction and others.

the potential and expressed capacity for physical, mental, and social activity during the phases of human life. Human beings, like other animal species, have a typical life course that consists of successive phases of growth, each of which is characterized by a distinct set of physical, physiological, and behavioral features. These phases are prenatal life, infancy, childhood, adolescence, and adulthood (including old age). Human development, or developmental psychology, is a field of study that attempts to describe and explain the changes in human cognitive, emotional, and behavioral capabilities and functioning over the entire life span, from the fetus to old age. Most scientific research on human development has concentrated on the period from birth through early adolescence, owing to both the rapidity and magnitude of the psychological changes observed during those phases and to the fact that they culminate in the optimum mental functioning of early adulthood. A primary motivation...